A digital image is represented by a 2D array with a size of the integer of width×height. Colors are represented by 3 arrays of different formats such as RGB, YUV and LUV and sometime the fourth channel is also used to represent a transparent region (alpha channel). The present disclosure only focuses on analyzing luminance data rather than color or transparency, that is called 1 channel representing luminance such as the Y channel of format YUV. In digital images, the most common integer data format (bit depth) is unsigned 8 bits/pixels (with a value from 0 to 255).
In a display panel, after gamma correction, the luminance value is converted to (cd/m2) with a range from the lowest Y=0 (darkest) level to the highest Y=2 bit depth −1 (brightest) level. The luminance range covered by the display panel is referred to as the dynamic range of the display panel.
Digital images may not use all available luminance levels. For example, an 8 bit/pixel image may use only Y values from 64 to 191, which only covers 50% of the entire range of 0 to 255. An image with low (large) luminance value coverage is called a low (high) dynamic range image (LDR or HDR image respectively).